Note: I am sadden to advise that Mr. Black has passed
away. His articles are left in tribute to his historical research. May he rest
in peace

The story begins on the 22nd December 1939 at a wartime
conference to discuss bombsight policy. A request was made by Air Chief Marshal
Sir Edgar Ludlow-Hewitt, Air Officer Commanding Bomber Command for a new
bombsight. He stated that the existing bombsights in service or
about to enter service, did not meet the requirements of his Command. He
wanted a bombsight that did not require such a long run up to the target and
gave his pilots an opportunity to engage in evasive action on the bombing
run.

His remark may have been prompted by the fact that four
days earlier, Wellingtons of his command took part in a disastrous raid upon
shipping off Wilhemshaven in which twelve of the twenty two
Bomber Command aircraft were shot down. This defeat was to have major
consequences for the conduct of WW2 as a whole. It marked the time when night
operations became the major policy of the RAF Bomber Command, the point when
radar was first used to detect raiding RAF bombers attacking main land Germany
and the birth of the Mk XlV bombsight.

Although later correspondence records the fact that the
AOC did make the request during this meeting, the detailed
minutes fail to record his request although the volunteering of Prof. Blackett
to design a sight to meet his needs was noted. Other documents of the time refer
to this request by Sir Edgar Hewitt and so the need for the highest security may
have lead for its omission from the minutes.

Through the influence of others present at the meeting
he quickly obtained laboratory and engineering facilities at Royal Aircraft
Establishment, Farnborough where he could design and initially
develop the new bombsight which he based upon the principles of the existing
CSBS Mk lX bombsight.

Professor Blackett was an eminent physicist of the time
and in his capacity as a scientific advisor to the Air Ministry
was present at the meeting. He had volunteered to design a bombsight that would
meet the requirements of A O C Bomber Command. Thus was born the Blackett
Bombsight. From that day forward, the proposed bombsight had an enthusiastic
reception from the RAF and Air Ministry officials before even the first
prototype was built.

His new bombsight proved to be the significant
improvement that RAF Bomber Command required. The proposal met in full the
requirements of the A.O.C Bomber Command.

It was first known as the Blackett sight and later as
the MkXlV. It is interesting to note that it was first
described as a medium altitude stabilised bombsight to be used within the 5,000
to 10,000 feet range. Later, the maximum altitude for which the sight was
designed was 20,000 ft and later still increased to 25,000 ft. The first
prototype known as the Mk Xll, consisted of the sighting head
only with a second member of the crew feeding data into the sight. This was
declared undesirable and resulted in a computer cabinet being designed and built
which allowed the sight to be operated by the bomb aimer only. This was
designated the MkXlV.

When the concept of the MkXlV had been
proved, Prof Blackett left the project team for other important work in Coastal
Command. Dr H.J.J. Braddick took his place
and was later nominated by Blackett as the co-inventor of the sight.

At an early stage in its trials at the Aeroplane and
Armament Establishment, Boscombe Down; it became recognised as
the first bombsight whether British, German or American which offered reasonable
accuracy in conjunction with a wide degree of tactical freedom. The
tachometric bombsights such as the ABS Mk2,
the later SABS and the American Norden with
their longer run-ups were preferably used at heights above 21,000 ft to reduce
the risk from anti aircraft fire.., This height that time was above the
maximum service ceilings of the new four engine bombers, the Halifax,
Stirling’s and
Manchester’s.

The searchlight and gunnery defences surrounding a vital
German target were often sited to take advantage of the short period when the
attacking bomber had to fly straight and level to use in order to use its sight
to accurately bomb the target. The shorter the run up to the target, the less
opportunity it gave the anti aircraft guns and searchlights to aim at the
bomber.

The new bombsight was required to be manufactured in
quantity to equip the new heavy and medium bombers in production for Bomber
Command. It was unique in that it could be built using relatively lower
grade labour than either the Norden or the British Automatic
bombsights. Both these bombsights required a
higher proportion of input from high grade instrument makers in their assembly
and led to considerable delays in the production of the Norden.
The redesign of the Norden for mass production resulted in the
introduction of significant errors which eventually affected the performance of
the sight in USAAF service.

The Mk XlV has another little known
advantage over its predecessor the ABS Mk ll, in that its bulk was smaller and
was less of an impediment to the view through the clear bombing panel by the
prone bomb aimer. A clear view through this panel was also important as it was
used for map reading. This problem was so serious that consideration was given
to providing with each bomber an alternative nose section with the front turret
omitted giving space for a larger bombing panel.

With the prototypes of the bombsight operating
satisfactorily, the enthusiasm continued for its future use in Bomber Command.
This contrasted with the reception of its predecessor, the Automatic Bombsight
that was received with little optimism for its eventual
success.

At an early stage in its development, steps had to be
taken to provide large quantities of the sights to meet the expansion of Bomber
Command. The decision was made therefore to sub-contract production of the
MkXlV to the USA, which would be additional to the output of
British companies and also gain access to a much larger pool of
labour.

Sperry Gyroscope Company of USA were approached by the
British Purchasing Commission in Washington. This company already had many years
experience in manufacturing bombsights which now included the
S-1 tachometric bombsight then in production for the
USAAF. This bombsight was designed to supplement the production
of the Norden, which at that time was in chronic short supply
for aircraft of the US Navy and United States Army Air Force and the subject of
much controversy between those services.

Mr Vose of Sperry Gyroscopes of America
expressed considerable interest in the MkXlV sight.
He claimed that his company would be able to redesign the sight for mass
production and in so doing, enhance its performance. He stated that at that time
his company had development staff immediately available to undertake this work
now that their Sperry-0-1 had largely completed development. He requested a set
of drawings to be made available to his company with a sample bombsight being
made available as soon as possible.

Before these drawings could be handed over the question
of the patents involved had to be considered. Professor Blackett became involved
once more with Dr Braddick. Dr Braddick of
whom little is now known, had also played a leading part in the development of
the SABS MllA and also the cancelled
SABS Mklll. This former sight was in final
stages of development around about the same time. Although manufactured in small
quantities, (less than 1,000) it was used very successfully by 617 Squadron in
its precision bombing. It was a further development of the generally unpopular
Automatic Bombsight.

Mr. Vose of Sperry, in making his case
for the manufacture under license of the MkXlV optimistically
implied that the USAAF would be interested in acquiring the
bombsight for use on some of its types of bombers. Later an old Hudson was
indeed equipped with a Mk XlV/T1 but the USAAF
rejected it because of the requirement to set manually, a wind speed and
direction and for lack of connection to the aircraft through the automatic
pilot. These requirements were a feature of the Norden and
Sperry bombsights.

The decision was made to subcontract the manufacture the
Mk XlV to Sperrys where the American version
would be known as the T1 bombsight.

When the initial development work had been completed by
Sperrys they in turn subcontracted the work to A.C Spark Plug
Company at their Flint plant in Michigan. This company was a division of General
Motors.

This decision was probably due to the pressure on their
Sperry USA factories in producing a large number of vital products for the
American expansion of their own armed services.

Throughout the contract, Sperry production engineers
supervised the production at the A.C Spark Plug plant. Some components such as
the gyros were supplied ready to install by Sperrys.

The section of industrial history which deals with the
development and production of equipment for the armed services of any nation is
very rarely recorded in any detail; company archives rarely survive for more
than a few years and are often not available to the historian. The A.C.
Sparkplug Company of Flint, Michigan was no exception to this
rule and the author has been very fortunate in being contacted by Mr George A.
Krepps in the USA who was an executive at the plant in WW2
throughout the period of the contract. His job was to supervise the quality
control functions relative to subcontracted incoming sub-assemblies, final sight
assembly and final test. His detailed recollections provide the
basis for this article.

The plant provided at that time a number of products to
the aircraft manufacturers and government defence departments such as sub-
assemblies for the Sperry S-1 Tachometric bombsight. They were
also building gun sights for fighter and bomber aircraft that were based upon
British designs subcontracted to the Americans. There were also, of course,
millions of spark plugs produced for use in both civilian and military internal
combustion engines.

The RAF Air Staff in agreeing to these arrangements
hoped that the American bombers produced in the USA would arrive in Britain with
the MkXlV/T1 already in installed in their
bombers.

Work on preparing the bombsight for mass production
started in May 1942 with the first T1 being ready for test in November that
year.

Initially, Sperry provided three production engineers to
work with A.C. Spark Plug engineers in the reworking of the drawings so as to
break the bombsight and sighting head mechanisms into sub-assemblies. They
suggested improved production methods and provided the criteria to be used.
Sperry took steps to ensure that test procedures and equipment met Sperry
standards by the training of staff and supervisors.

Changes were made in the overall design of the computer
cabinet to reduce the number of manufacturing operations required.
The tubular frame which surrounded the computer cabinet was modified to reduce
the amount of welding necessary.. All threads used on components were
changed to American standard threads. Die castings were extensively used where
ever possible in the sub-assemblies. Sintered oilite sleeve
bearings replaced ball bearings. The higher standard machine and cutting tools
available to the Americans further improved the
output.

When it appeared to Sperrys that A.C.
Spark Plug had the contracts under control, they arranged with the British
Purchasing Commission in Washington to place further orders directly with A.C
Spark Plug.

The twelve main sub- assemblies were manufactured by
12-15 sub-contractors of diverse backgrounds such as the manufacturers of
pinball machines, cameras, typewriters, and thermostats. A high proportion of
female labour was employed, reaching 80%. With so many diverse sub-contractors
involved, careful inspection procedures had to be put in place by Sperry. In
particular, inspection procedures had to ensure the correct alignment of the
sub-assemblies prior to final assembly at the Flint
Plant.

At the Flint plant, an assembly line was installed from
incoming post inspection stores with a ‘line drop’ at each
assembly station for fitting the scheduled
assemblies.

The final testing of the completed units was carried out
in five vacuum chambers approximately 900 x 1200 x 900 mm. In these chambers
could be simulated the bombing operation with climb, glide, air speed and cross
wind direction. Shaft outputs for the sighting heads were also recorded.
The bombsight was tested at 1000 ft levels to 20,000
ft.

The sighting head final test comprised mounting the head
on an elevated platform to about 2.4 m above floor level. The input into the
sighting head from the computer cabinet was simulated together with specified
limits of roll. The performances of both sighting head and computer
cabinets were checked to meet the specification. The figures were recorded
on charts supplied with each computer cabinet. After final testing the finished
computer cabinet or sighting head was mounted in a steel cabinet ready for
dispatch.

In August 1943 Sperry published a technical manual
describing in considerable detail the T1 bombsight which is now in the
possession of the author. The manual recorded the design at that one point in
time; when the gyros were air suction driven. In 1944, and from Serial No
18,000, Sperry electrically driven units replaced the air suction gyros. These
were used in all succeeding developments of the T1.

The manual states that separate versions were produced
were as follows;

In this early manual, apart from a line drawing of a
single Lancaster there were no references to the Halifax, Lancaster, Manchester
or Stirling’s heavy bombers, yet these aircraft were to be the
principal bombers of Bomber Command. There is a mystery surrounding this
statement, as I am assured by George Krepps that A.C Spark Plug
produced in August 1944 one model only of the bombsight. Thus technical centres
in USA and UK would have set up the bombsight to suit the flying characteristics
of aircraft designated to use it.

A final version of the bombsight was developed to
cater for all aircraft that was designated to receive this instrument. A set of
loose cams were added to each sight produced which included a dedicated cam for
each type of aircraft to using the T1 bombsight. The dedicated cam automatically
set into the bombsight the flying characteristics of the aircraft .

No attempt was made to pair up a sighting head with a
particular computer cabinet. These were to remain completely interchangeable
with the corresponding British units as manufactured in Britain. In fact, an RAF
bomber aircraft may well have had a T1 computer cabinet, operating quite
effectively without loss of accuracy, with a Mk XlV sighting
head or vice-versa.

An astonishing total of 23,450 T1 bomb sights were
produced at the Michigan plant over the period of November 1942 until June/July
1945. When the factory was fully operational, the output reached about 54
completed bombsights per day, using two shifts of workers eight
hours per day in a six day week.. From a pre-war figure
of 5,000 employees at A.C. Spark Plug the total size of the operation grew to
20,000.

Further development of the MkXlV sight
in Britain appears to have been based on the T1 series of
bombsights rather than the British version.
Towards the end of the war bomber aircraft were flying higher
and so the bombsight design had to be modified to allow for this
change. In December 1944 the T1A and B versions were produced. These
bombsights had a similar speed range but the effective height
range was increased to 25,000 ft.The maximum climb the sight could measure had
been increased from 5° to 11° and 12° respectively for yet another
version. The T2 and T4 sights were developed and manufactured in Britain
after the war for later aircraft and often installed in conjunction with radar
devices. With the T2 the effective height range was increased to
600-25,000 ft, a speed range of 150-350 knots and a wind speed of 90
knots. The higher wind speed was to allow for the jet stream at greater
heights.

It now seems that MkXlV was the name
universally used by RAF and Commonwealth Air Bombers for the bomb sight and most
were unaware of the T1 version. However, the version most readily found today in
museums and in private collections will be that of the T1 and its
derivatives.

During the time the contract was executed it appears
there was no record of a visit to the factory by RAF representatives to
demonstrate the considerable operational value of the T1 to the Allied War
effort. This was an omission that was regrettable. However in August 1943 Mr
George Mann visited the UK for a period of about a year liasing
on behalf of A.C Spark Plug with RAE Farnborough,
Boscombe Down, and the Ministry of Aircraft Production. He also
toured unnamed British factories manufacturing MkXlV
bombsights.

When the contracts were completed George
Krepps continued his association in 1953 with
bombsights by working on the Bombing and Navigational Systems
for the American B52 bomber.

The author would like to place on record his
appreciation of Mr Geoge Krepps in
providing extensive information in the manufacture of the T1 version of the
bombsight in the USA and to Mr D.W. Allen in granting access to his unpublished
autobiography notes on his part in the development of the Mk
XlV bomb sight at RAE Farnborough in 1941